Alternate Definitions for solar activity

Transient perturbations of the solar atmosphere as measured by enhanced x-ray emission (see x-ray flare class), typically associated with flares. Five standard terms are used to describe the activity observed or expected:

Very low x-ray events less than C-class.
Low - C-class x-ray events.
 
Moderate - isolated (one to 4) M-class x-ray events.
High - several (5 or more) M5 or greater x-ray events.